1. Field of the Invention
This invention relates generally to hand-held pipettes employing axially reciprocating pistons to aspirate and dispense fluids into and out of replaceable pipette tips, and is concerned in particular with an improved actuator assembly and associated system for automatically controlling the stroke of such pistons.
2. Description of the Prior Art
Hand-held pipettes with manually driven pistons and automatic stroke control mechanisms have been known for nearly a decade. FIG. 14 illustrates one such pipette 10 developed in 1994 by engineering students at Northeastern University in Boston, Mass. The pipette 10 includes a piston 12 having its lower end received within a cylinder 14 configured at its distal end to accept a pipette tip (not shown). The piston 12 is acted upon by a manually operable plunger shaft 16. A collar 18 on the plunger shaft is resiliently urged against a rear stop 20 by a spring 22 acting on the piston 12. The piston is advanced into the cylinder 14 by manually depressing the plunger shaft 16 against the biasing action of the spring 22. A forward stop 24 is engageable by the collar 18 to limit the extent to which the plunger shaft can be depressed. The piston stroke “S” is thus defined by the distance between the forward and rear stops 20, 24.
The rear stop 20 forms part of a frame 26 slidably mounted on a guide shaft 28 supported by the housing in parallel relationship to the plunger shaft 16. A stepper motor 30 has its output screw shaft 32 threaded through an upper part of the frame 26. The motor is operable to automatically shift the frame 26 along the guide shaft 28, resulting in a corresponding adjustment of the rear stop 20 and a corresponding adjustment to the stroke of the piston 12.
One problem with this type of automatic stroke adjustment is that when advancing the rear stop 20 towards the forward stop 24 in order to reduce the length of the piston stroke, the motor 30 must work against a gradually increasing biasing force being exerted by the spring 22. Thus, the motor either must be sized large enough to overcome this biasing force, or the plunger shaft 16 must be depressed to unload the rear stop prior to making any stroke adjustment. Larger motors contribute disadvantageously to the size and cost of the unit, whereas the need to preliminarily unload the rear stop unduly complicates the stroke adjustment sequence. Larger motors also consume more power, thus requiring larger batteries, which further adds to the size and weight of the unit.
Another problem stems from the fact that the initial or “starting” force required to depress the plunger shaft 16 will vary, depending on the extent to which the spring 22 has been compressed in response to prior adjustments of the rear stop 20. Such variations in starting force can distract laboratory personnel from the task of precisely aspirating and dispensing fluids.
The parallel arrangement of the plunger shaft 16 and motor output shaft 32 also contributes disadvantageously to the overall size of the housing and hence the weight of the unit, making it more expensive to manufacture and less convenient to use.
The present invention has as its overall objective the provision of a hand-held manually-driven pipette incorporating an improved stroke adjustment mechanism that obviates or at least substantially minimizes the above described problems.